(DiamondLightSource/hyperion#1307) change DetectorParams detector_siz…

…e_constants validator to allow the declared type

src/dodal/devices/detector/detector.py

@@ -67,7 +67,11 @@ def create_beamxy_and_runnumber(cls, values: dict[str, Any]) -> dict[str, Any]:
     def _parse_detector_size_constants(
         cls, det_type: str, values: dict[str, Any]
     ) -> DetectorSizeConstants:
-        return constants_from_type(det_type)
+        return (
+            det_type
+            if isinstance(det_type, DetectorSizeConstants)
+            else constants_from_type(det_type)
+        )
 
     @validator("directory", pre=True)
     def _parse_directory(cls, directory: str, values: dict[str, Any]) -> str: